Structures on masks or substrates are normally measured using mask or wafer structure size test sets (for example CD-SEMs (SEM: Scanning Electron Microscopy)). In the previously normal measurement methods, the measurement accuracy (performance or test equipment capability) of the test set is determined and adjusted regularly with the aid of a specific calibration measurement. The calibration measurement is carried out independently of and separately from the actual measurement on the product, that is to say on the mask or on the substrate. Calibration measurements such as these may be a part of an SPC method (SPC: Static Process Control). In the already known calibration methods, measurement error limits are preset, which the mask or wafer structure size test set must not exceed. If the mask or wafer structure size test set nevertheless exceeds the measurement error limit, then the test set is recalibrated. Recalibration is thus carried out only when a predetermined error limit is exceeded; a certain residual error up to a predetermined maximum size is thus permissible. These “permissible” residual errors in the already known measurement methods are included directly in the structure size measurement of the structures located on the product (mask or substrate).